Thermoforming is a manufacturing process where a plastic sheet is heated to a pliable forming temperature, formed to a specific shape in a mold, and trimmed to create a usable product. The sheet, or “film” when referring to thinner gauges and certain material types, is heated in an oven to a high-enough temperature that it can be stretched into or onto a mold and cooled to a finished shape. In its simplest form, a small tabletop or lab size machine can be used to heat small cut sections of plastic sheet and stretch it over a mold using vacuum. This method is often used for sample and prototype parts. In complex and high-volume applications, very large production machines are utilized to heat and form the plastic sheet and trim the formed parts from the sheet in a continuous high-speed process, and can produce many thousands of finished parts per hour depending on the machine and mold size and the size of the parts being formed.
Thermoforming differs from injection molding, blow molding, rotational molding, and other forms of processing plastics. Thin-gauge thermoforming is primarily the manufacture of disposable cups, containers, lids, trays, blisters, clamshells, and other products for the food, medical, and general retail industries. Thick-gauge thermoforming includes parts as diverse as vehicle door and dash panels, refrigerator liners, utility vehicle beds, and plastic pallets. In the most common method of high-volume, continuous thermoforming of thin-gauge products, plastic sheet is fed from a roll or from an extruder into a set of indexing chains that incorporate pins, or spikes, that pierce the sheet and transport it through an oven for heating to forming temperature. The heated sheet then indexes into a form station where a mating mold and pressure-box close on the sheet, with vacuum then applied to remove trapped air and to pull the material into or onto the mold along with pressurized air to form the plastic to the detailed shape of the mold. After a short form cycle, a burst of reverse air pressure is actuated from the vacuum side of the mold as the form tooling opens, commonly referred to as air-eject, to break the vacuum and assist the formed parts off of, or out of, the mold. A stripper plate may also be utilized on the mold as it opens for ejection of more detailed parts or those with negative-draft, undercut areas. The sheet containing the formed parts then indexes into a trim station on the same machine, where a die cuts the parts from the remaining sheet web, or indexes into a separate trim press where the formed parts are trimmed. The sheet web remaining after the formed parts are trimmed is typically wound onto a take-up reel or fed into an inline granulator for recycling.
Each polymer has different properties. Not every polymer is suitable for thermoforming. Amorphous polymers are preferred, melt temperatures are critical, physical properties significantly impact the resulting products. The most common materials that are used are thermoplastics such as polypropylene.
There is a need for biodegradable, strong and flexible polymers that can be processed by thermoforming. WO 2007/092417 to Rizk discloses compositions of polylactic acid (“PLA”) toughened with P4HB that have desirable properties for thermoforming, but PLA is relatively inflexible. WO 2011/071666 to Wel discloses methods to make thermoformable compositions of PHA and PLA by crosslinking, including P4HB crosslinked with PLA. WO 2009/137730 to Uradnisheck discloses PHA compositions for thermoforming, including PHAs containing 4-hydroxybutyrate monomers, methods to thermoform PHAs with glass transition temperatures of approximately 55° C. and melting points of around 155° C., and methods to heat-treat finished articles at 105° C., or methods to heat-treat the low melting point P4HB polymer and thermoforms of P4HB. U.S. Pat. No. 5,834,582 to Sinclair discloses compositions of degradable materials and nontoxic modifiers that can be thermoformed. EP 1136510 to Datta and Siegmann discloses a very long list of bioabsorbable polymers, including a polymer of hydroxybutyrate (the common name for poly-3-hydroxybutyrate), which can be thermoformed.
P4HB has entirely different properties compared to P3HB (“PHB”), PLA and copolymers thereof. For example, PHB has a melting point of 180° C. versus a melting point of about 60° C. for P4HB. The polymers also have substantially different glass transition temperatures and mechanical properties. P4HB has a glass transition temperature of −55° C. PHB is a relatively hard brittle polymer with an extension to break of just a few percent, whereas P4HB is a strong extensible polymer with an extension to break of about 1000%. P4HB has strikingly different uses from P3HB, and copolymers of P4HB with polymers such as PLA and P3HB, as well as significantly different processing requirements.
Thus, there is currently no disclosure of how P4HB can be thermoformed, the properties of P4HB films or sheets necessary for thermoforming, the intrinsic viscosities of P4HB that can be processed by thermoforming, the loss of intrinsic viscosity upon thermoforming P4HB, the conditions necessary to thermoform P4HB, the properties of P4HB thermoforms, such as tensile strength, burst strength, elongation to break, bending strength, and tensile modulus, produced by thermoforming, or the benefits of thermoforming P4HB. There is also no disclosure of the use of P4HB thermoforming to produce laminates with desirable properties.
It is therefore an object of the present invention to provide compositions of P4HB that can be thermoformed.
It is another object of the present invention to provide a means of thermoforming P4HB.
It is a further object of the present invention to provide thermoforms of P4HB produced by thermoforming characterized by specific physical properties.
It is still another object of the present invention to provide thermoforms of P4HB produced by injection molding with enhanced mechanical properties and controlled degradation profiles that can be used in medical applications.
It is yet a further object of the invention to provide laminated thermoforms with P4HB films and sheets, and methods to produce such laminates.